This invention relates to a method and apparatus for controlling a PWM (pulse width modulation) inverter, in which higher harmonic components included in the output currents of the PWM inverter driving an AC motor are reduced so as to reduce magnetic noise (acoustic noise) generated from the AC motor.
An instantaneous-value control system and a mean-value control system are generally used for the control of a PWM inverter. According to the instantaneous-value control system, turn-on control of the PWM inverter is made by PWM pulse which is modulated on the basis of differences of magnitude and polarity between a current command pattern signal (a sinusoidal wave signal), and a detected current signal at the inverter output. This instantaneous-value control system is advantageous over the mean-value control system. In the mean value control system, a voltage command pattern signal is used as a modulation signal and is compared with a carrier wave signal (a triangular wave signal) to generate a PWM pulse signal for each phase. The current control response of the former system is better than that of the latter system. Practical applications of the instantaneous-value control system having such an advantage to the control of PWM inverters have been attempted in recent years in this field, and some of the attempts have been proved successful and put into practical use.
It is commonly known that an AC motor driven by a PWM inverter generates magnetic noise (accoustic sound). Such magnetic noise is generated from the AC motor for the reason that higher harmonic components are included in the output currents of the PWM inverter. Especially, when the instantaneous-value control system is used for the control of the PWM inverter, magnetic noise giving a feeling of discomfort is generated from the AC motor during rotation of the motor in a low-speed range.
On the other hand, such an AC motor is now frequently installed not only in a place of high noise level but also in a place of low noise level. Even when an AC motor driven by a PWM inverter is installed in a place of high noise level, generation of noise giving a feeling of discomfort is not desirable in view of the working circumstance. It is therefore strongly demanded to reduce the undesirable magnetic noise generated from the AC motor driven by the PWM inverter.
Various methods have been proposed hitherto for reducing the magnetic noise generated from an induction motor driven by a PWM inverter. In one of the known methods, the magnetic flux produced in the induction motor is weakened in a light-loaded or no-loaded operating condition of the induction motor, and, in another known method, both of the instantaneous-value control system and the mean-value control system are incorporated to be switched over between each other depending on the rotation speed of the induction motor, as described in, for example, the following documents:
(A) Japanese Patent Application Laid-open No. 56-83284 (1981) entitled "Apparatus for variable-speed operation of induction motor". The document (A) describes that, when an induction motor is driven by a PWM inverter, the magnetic flux produced in the induction motor is weakened in a light-loaded or no-loaded operating condition of the motor so as to reduce magnetic noise generated from the motor.
(B) Japanese Patent Application Laid-open No. 56-117577 (1981) entitled "Inverter control apparatus". The document (B) describes that, when an induction motor is driven by a PWM inverter, the mean value of the inverter output currents is controlled in a low frequency range of the inverter output, while the instantaneous value of the inverter output currents is controlled in a high frequency range of the inverter output, so as to reduce magnetic noise generated from the motor during rotation in its low-speed range.
However, the former proposal is defective in that magnetic noise cannot be reduced in the rated-load operating condition of the motor when the magnetic flux density attains its rating, although such magnetic noise can successfully be reduced in the no-loaded (or light-loaded) operating condition of the motor. Also, the latter proposal is defective in that the current control response is lowered in the operation range (the low-speed operation range) controlled by the mean-value control system, and that a torque variation occurs at the time of switch-over between the control systems.